Place pesto, buttermilk, and cucumber into a blender. Puree until smooth. Transfer to an airtight container and chill until ready to serve. To serve, pour equal amounts into glasses or cups and garnish with extra basil.
Notes
Makes Four 8-ounce or eight 4-ounce servings.
Recipe by Cafe Johnsonia at https://cafejohnsonia.com/2014/06/chilled-pesto-cucumber-buttermilk-soup.html